Food proteins and gut mucosal barrier I. Binding and uptake of cow's milk proteins by adult rat jejunum in vitro.

To study small intestinal binding, breakdown, and uptake of bovine serum albumin (BSA) and beta-lactoglobulin ( BLG ), radiolabeled with 125I or 14C, the rat everted gut sac technique was used as a model of intestinal protein handling. Binding and uptake were found to be nonspecific and nonsaturable. There was a close correlation between both processes (r = 0.8). A consistent difference between BSA and BLG was shown in that more BLG was bound and taken up between 1 and 30 min than was BSA (P less than 0.001). As shown by trichloroacetic acid precipitation and gel filtration on Sephadex G-100 and Bio-Gel P-2, this difference was due to differential protein breakdown that takes place during protein attachment and transport across the intestinal mucosa. High-molecular-weight fragments were generated from BSA, whereas the molecular weight of most fragments generated from BLG was very low (mol wt less than 500). Size and structure of these food protein fragments might not only influence their intestinal binding and uptake characteristics but might also play a role in the physiological induction of tolerance and their relative antigenic potential.